You Have One Job: Compared to Multi-Tasking Workers, Soldier Ant Brains Small

3-D reconstructions of Eciton burchellii parvispinum heads with the brain and specialized structures highlighted. The head on the left is a worker, and the one on the right is a soldier. Courtesy of Sean O'Donnell.

Brains and brawn apparently don’t mix in army ant soldiers, a new study shows.

Why that occurs might have to do with how much energy it takes to develop and power brain tissue, according to O’Donnell and his co-authors.

“The patterns we saw suggested natural selection favors efficient allocation of brain tissue, with the result of reduced investment in the behaviorally specialized soldiers,” O’Donnell said.

In many ants, individuals in a colony are divided into different “castes,” meaning group members play different roles to help the colony survive. O’Donnell chose to study ants in the Eciton genus because of the stark body differences between castes; two of the major roles individuals take are workers or soldiers.

Soldier ants are larger, stronger and have powerful mandibles that work like pincers to fight off predators threatening the colony. That is effectively their only job.

Workers, meanwhile, have smaller bodies and mandibles more suited for grabbing and moving things. Their jobs are more varied, ranging from hunting and collecting food to tending to the young.

A group of Eciton burchellii cupiens ants in Ecuador. The grey arrow points to a foraging worker and the right points to a soldier. Courtesy of Sean O'Donnell.

As such, it follows that worker ants could have larger brains compared to their body size. With so many different tasks, they need the accompanying brainpower to accomplish them.

Moreover, the study found that some specific parts of their brains — including the areas related to learning and smelling — were also comparatively larger in workers than soldiers. The soldiers’ brains could actually be described as a little more “Spartan,” with the special areas more trimmed down.

So, does that mean soldier ants are dumber than workers?

“I’m not sure I’d say dumber so much as simpler,” O’Donnell said. “That’s our assumption, in a way, not our conclusion. Our idea was that soldiers’ simple behavior might require less brainpower.”

In this case, “power” is the important word. Since energy is so precious in ant colonies, they must conserve it to survive. And with brains needing so much energy to power, the researchers theorized that the ants with fewer tasks would be given less of a share of that energy.

Basically, the idea is that colonies are favored to spend their limited energy sources as prudently as possible, evolutionarily speaking.

“Colony-dwelling animals potentially provide a unique window on how brains and behavior evolve,” O’Donnell said. “An individual animal’s behavior has less relevance to itself, evolutionarily, and more relevance to its colony or group. Natural selection can occur by colony performance, and colony evolution may shape individual animals’ brain investment.”

Seeing this in action in Ecitonants provides a glimpse of how things may work for others.

“This leads to new predictions about patterns of brain investment in social animals,” O’Donnell said.